This invention relates to physiological measuring instruments and in particular to a body-mountable instrument for sensing and measuring heart beat. This invention further relates to U.S. Pat. No. 4,312,358, which is assigned to the assignee of the present invention.
Instruments for measuring physiological parameters such as heart beat and the like are known in the art. Some of these instruments employ electrocardiogram (EKG) electrodes to sense heart beat, as exemplified by U.S. Pat. No. 3,792,700. EKG pick up electrodes, however, are often cumbersome and inconvenient to use, particularly for persons engaged in physical exercise. Other physiological measuring instruments use pressure sensors to detect pulsing of blood and transducers to convert such pressure pulses to electrical pulses, as exemplified by U.S. Pat. No. 3,742,937. Pressure sensors are typically more conducive to portable, miniature heart beat measuring instruments than EKG electrodes because only a single sensor is required instead of two separately disposed electrodes.
A major problem associated with prior art heart beat measuring instruments is the problem of accurately measuring the heart beat of one who is engaged in vigorous physical activity because of the background noise associated with body movement during exercise. Periodic body motion associated with the user's feet hitting the ground and his arms swinging back and forth during exercise will cause the user's blood to pulse, particularly in extremities such as the hands and feet, and will be detected by a pressure sensor in much the same way as a heart beat, thereby tending to obscure the true heart beat.
Prior art attempts to deal with this problem have generally involved subtracting an electrical signal indicative of body movement generated by a first sensor from a composite signal indicative of heart beat plus body movement generated by a second sensor to determine the true heart beat. Such a technique is described in U.S. Pat. No. 4,063,551.
This technique, however, often yields erroneous heart beat information because the amplitude and phase of the body movement signal vary depending upon the body location where it is detected. For example, if a pressure sensor is placed on the user's finger to sense the pulsing of blood caused by the user's heart beat and body movement and an accelerometer sensor is placed on the user's wrist to sense only body movement and the user is running or jogging, the natural motion of the user's arm during running or jogging will cause blood to accumulate in the finger so that the amplitude of the body movement signal detected on the finger is greater than that detected on the wrist. Therefore, if the body movement signal detected on the wrist is subtracted from the composite heart beat plus body movement signal detected on the finger, differences in the amplitudes of the body movement components in the two signals will cause spurious peaks to appear in the resultant heart beat signal, which may be mistaken for actual heart beats. Thus, it will appear that the user's heart is beating faster than it actually is. Since the amplitude of the body movement signal detected on the finger can be two to three times that of the body movement signal detected on the wrist, errors in heart beat information can be significant.